1. Field of the Invention
The present application relates to compression techniques and, more particularly, improved compression techniques for use with the mixed raster content compression format.
2. Description of the Related Art
Image compression addresses the problem of reducing the amount of data required to represent a digital image. Compressed data (i.e., text and/or image data) may be easier to transmit by, for example, facsimile and/or e-mail than non-compressed data.
The underlying basis of the reduction process is the removal of redundant data during a scanning operation or, alternatively, from existing electronic files. Compression techniques typically fall into two broad categories: information preserving compression (i.e., lossless compression) and lossy compression. Lossless compression allows data to be compressed and decompressed without the loss of information. Lossy compression provides higher levels of data reduction as compared to lossless compression but results in a less than perfect reproduction of the original image (i.e., information loss).
Lossless compression methods (e.g., Lempel-Ziv or LZW) perform well on text but do not perform particularly well on scanned images. In contrast, lossy compression methods (e.g., JPEG) work fairly well on continuous-tone pixel arrays but do not work particularly well on text.
Accordingly, the mixed raster content (MRC) format may be used to compress documents including both text and images. The MRC format segments a document into two or more planes or layers. For example, each pixel in a document may be segmented into a text layer, a foreground layer and a background layer, as described in U.S. Pat. Nos. 5,778,092 and 6,324,305, the entire contents of which are incorporated herein by reference. The text layer may include only text data of the document, the foreground layer may include color information of the text data and the background layer may include any image that is not considered text data.
Once each pixel has been segmented between two or more layers, each layer may be separately compressed using an appropriate compression method based on the content of the layer. For example, the text layer may be compressed using a lossless compression method such as LZW while the foreground and background layers may be compressed using a lossy compression method such as Joint Photographic Experts Group (JPEG).
In the prior art, the segmentation step may be carried out by separating high frequency pixels, which typically are associated with text, from low frequency pixels, which typically are associated with image and background data. Such edge detection techniques often misclassify pixels belonging to large halftone dots (i.e., halftone dots having a size equal to or greater than the edge detector kernel) as high frequency pixels and therefore treat them as text pixels, thereby increasing the size of the compressed file.
Accordingly, there is a need for a segmentation technique having an improved capability for detecting the difference between halftone areas and text areas.